VolcanismChapter 16

By: Marshall Hartung and Allen Zhu

● Lava - molten or semifluid rock erupted from a volcano or fissure● Magma - fluid or semifluid material below or within the earth's crust from which lava and other

igneous rock are formed by cooling● The primary factor that determine the characteristics of a lava flow is composition● Composition of lava:

○ Felsic ■ High percentage of silica; low percentage of iron and magnesium■ Ex: rhyolite and granite

○ Intermediate■ Mid percentage of silica; mid percentage of iron and magnesium■ Ex: andesite and diorite

○ Mafic■ Low percentage of silica; high percentage of iron■ Ex: basalt and gabbro

Volcanic Deposits

Volcanic Deposits - Lava Flow● The more mafic the lava, the more fluid the lava & the longer the lava remains fluid

○ Basaltic lavas can flow at velocities ≥ 100 km/hr (60 mph)● The composition of magma affects its boiling point & eruption temperature

○ Basaltic magma: 1000 - 1200 °C○ Felsic magma: 800 - 1000 °C

● Basaltic lava flows have two surface and internal features○ Aa - slow moving lava whose thick skin is broken into a rough, jagged surface○ Pahoehoe - highly fluid lava that spreads in ropey sheets○ The reason these lavas differ is because aa has lost its volatiles and consequently its fluidity○ Lava flows typically begin as pahoehoe and end as aa

Volcanic Deposits - Lava Flow● Pillow Lava - piles of ellipsoidal, saclike blocks that are characteristic of underwater eruption of

basalt and andesite

Volcanic Deposits - Lava Flow● Lavas are characterized by their textures and other features

○ There is an inverse relationship between the length of cooling time and crystal time■ The longer the rock cools, the larger the crystals and vice versa

○ Vesicle - gases are released from cooling lava, which forms ovoid gas cavities○ Pumice - an extremely vesicular and generally rhyolitic rock that contains lots of vesicles○ Xenolith - a foreign inclusion picked up by a magma that provides important clues about the

source region and the rock formations through which the rock passed en route to the surface○ Joints - when lava cools and contracts, shrinkage cracks often form○ Columnar jointing - joints that form perpendicular to the cooling surface○ Lava Caves - a tube that forms when the source of an enclosed stream of an enclosed stream

of lava is cut off and the remaining lava drained from the channel○ Spatter Cones - steep-sided, conical hills built from the spatter of lava fountains spewed out

of vents

Pumice

Lava Tube

Xenolith

Columnar Jointing

Volcanic Deposits - Pyroclastic Deposits● Water and dissolved gasses are important constituents of magma; when magma rises close to the

surface and the pressure drops, the volatiles may be released with explosive violence○ This is more common in the felsic, volatile-rich, viscous rhyolitic and andesitic lavas than in

basalts, which are more fluid and release volatiles more quickly● Pyroclasts - the fragmentary volcanic rock materials that are ejected into the air; classified by size

○ Dust - finest fragments; up to 2 millimeters in size○ Ash - sand sized fragments; 2 millimeters to 6 centimeters in size○ Bombs - largest fragments; larger than 6 centimeters

■ Typically ellipsoidal, discoidal, and irregularly rounded○ When pyroclasts fall, they build up deposits near their source○ When the fragments become cemented together (lithified), the rocks formed from the smaller

fragments are called volcanic tuffs; those formed from the larger ones are called volcanic breccias

Volcanic Breccias

Volcanic Tuff

Volcanic Bomb

Pyroclastic Rock

Volcanic Deposits - Pyroclastic Deposits● Nuée Ardente - a volcanic eruption that occurs when hot ash, dust fragments, and gases are

ejected in a glowing cloud that rolls downhill with amazing speed. The solid particles are actually buoyed up by the hot gasses, so that there is little frictional resistance to the pyroclastic flow.

● In 1902 a nuée ardente with an internal temperature of 800°C flowed down from Mont Pelée at a speed of 160 km/hr (100 mph) and killed ~28,000 people

● Typically, the fragments left behind a nuée ardente are still hot and soft, so when deposited and become compacted and stuck together to form ignimbrites.

Ignimbrites

Nuée Ardente

Eruptive Styles● Flood Basalt - the result of a giant volcanic eruption or series of

eruptions that covers large stretches of land or the ocean floor with basalt rock

○ Ex: the Columbia Plateau○ Comes from more mafic lava

● Phreatic Eruptions - pyroclastic eruptions which include violent steam blasts

○ Ex: Krakatoa and Mount Pelée○ Comes from more felsic lava

Eruptive Styles - Fissure Eruptions● Either lava (basaltic) or pyroclastic (felsic) materials● From long, narrow fissures● Basaltic lava

○ More fluid, so it flows away from source to form flood basalts and plateau basalts■ Ex: Columbia Plateau, Deccan Plateau, and the Parana Plateau

○ Associated with seafloor spreading● When fissures occur along the ridge crests of the seafloor, it forms rift valleys

○ The upwelling basalt frequently produces ridges centered on the deep valleys of the fissures rather than plateaus which form on land

● Iceland is slowly growing due to the mid-Atlantic rift widening● Dike - a nearly vertical column of basalt ● Fissure eruptions of pyroclastic materials rather than lavas are more likely when the parent magma

is more felsic than basalt○ Produce extensive ignimbrite sheets○ Ex: Great Basin, Yellowstone National Park

● Central Vent - the pipe or tube from which the extrusive materials issue; gives rise to the familiar cone shape of volcanoes

○ Craters form at the summits of most volcanoes and are usually centered over the vent● Basaltic lava forms wide volcanoes

○ Fast-flowing fluid forms a broad shield volcano○ Probably begin as fissures and develop central vents later○ Ex: Mauna Loa

● Felsic lava forms tall volcanoes○ Slow-moving, viscous fluid forms a volcanic dome○ Cinder Cone - a cone formed around a volcanic vent by fragments of lava thrown out during eruptions○ Ex: Mount St. Helens

Eruptive Styles - Central Eruptions

● Composite Cone Volcano - when a volcano emits lava as well as pyroclasts and is built of alternating lava flows and beds of pyroclasts

○ Ex: Fujiyama, Vesuvius, and Mount Etna● Calderas - large, basin-shaped depressions of volcanic origin

○ They are collapsed features that form after the violent ejection of large volumes of magma from a magma chamber several kilometers below the surface

○ Fresh magma can reenter the collapsed magma chamber to reinflate○ In a constant cycle of eruption, collapse, and resurgence○ Ex: Yellowstone caldera, Long Valley caldera, Valles caldera, Toba caldera, Kilauea, Crater

Lake, Krakatoa, and Olympus Mons● Diamtreme - a volcanic vent, or pipe, that is filled with breccia as a result of the explosive escape of

gases from the deep interior○ Produced by the upward movement of materials from great depths○ Formed by gas-charged magmas that melt their way to the surface, finally ejecting gases, lava

fragments from the vent walls, and fragments from the deep crust and mantle - all with explosive energy

Eruptive Styles - Central Eruptions

Yellowstone Caldera

Volcano Anatomy

Eruptive Styles - Other Volcanic Phenomena● When a nuée ardente meets a river, it can be transformed into a landslide or mudflow of volcanic

debris, called a lahar○ Can also be produced when the wall of a crater lake breaks, releasing water suddenly○ Can also be produced when glacial ice is melted by lava flows or nuée ardente

● Volcanic gases○ Water vapor is the main constituent (70-95%), followed by carbon dioxide, sulfur dioxide, and

traces of nitrogen, hydrogen, carbon monoxide, sulfur, and chlorine○ These gases formed the oceans and atmosphere billions of years ago○ The gases may be derived from deep within the Earth and may be making its way to the

surface for the first time (juvenile gas); some may be recycled groundwater and ocean water, atmospheric gas, or gas that has been trapped in rocks

● Geyser - a hot-water fountain that spouts intermittently with great force○ Ex: Old Faithful in Yellowstone Park

● Fumaroles - vents that emit gas or steam

Geyser

Fumarole

Lahar

● Kilauea - massive shield volcano that makes the large island of Hawaii○ The eruption of 1959-1960 was predicted using Tiltmeters

■ Instruments that measure tilting of the ground to observe swelling and deflation due to the accumulation and underground movement of magma

○ The upward migration of the magma could be traced by weak seismic disturbances originating at depths of 5-15 kilometers

○ Harmonic Tremors - the release of seismic energy that typically accompanies the movement of lava

○ The basaltic magma erupted from Kilauea is from the partially molten asthenosphere● Mount Pelée - a 1902 nuée ardente eruption in the Caribbean that devastated an entire town

○ Discussed earlier and in depth in the other presentation● Krakatoa - the massive 1883 volcanic eruption in Indonesia that generated massive tsunamis and

blocked out the sun○ Discussed in depth in the other presentation

● Mount St. Helens - a 1980 eruption in Washington that caused a lahar○ Discussed in depth in the other presentation

Case Histories of Volcanoes

Mount St. HelensKilauea

Krakatoa Mount Pelée

Patterns of Volcanism - Magmas● Mafic magmas (basalt)

○ Oceanic crust○ From peridotite in asthenosphere

● Felsic magmas (rhyolite)○ Continental crust○ Melt felsic granite continental crust○ Convergence zone with continent

● Intermediate magmas (andesite)○ Between felsic and mafic○ Subduction zones

Patterns of Volcanism - Ocean Ridge● Fracture between divergent

plates in asthenosphere● Ocean ridges, volcanoes,

seafloor, plateaus

Patterns of Volcanism - Intraplate● Aseismic ridges and other volcanic centers

○ From stationary hot spots within mantle■ Plate movement

● Guyots○ From spreading centers○ Erosion and lithosphere cooling

● Volcanoes within continents○ From basalt - no contamination○ Continental breakup

Patterns of Volcanism - Convergence Zone● Volcano chain parallel to deep sea trench● Island arc

○ Oceanic plate-oceanic plate○ Philippines, Japan, Marianas,

Aleutian Islands● Mountain chain

○ Oceanic plate-continental plate○ Cascades, Andes

Volcanism and Human Affairs● Devastative● Resurgent calderas● Precautions

○ Prediction■ Earthquakes, tilt, temperature change,

gas, gravity and magnetic field shifts○ Prevention

● Origins of oceans and atmosphere● Geothermal energy